Harmonic Convergence is a supernatural phenomenon that occurs once every ten thousand years. When the planets align, spiritual energy is greatly amplified, causing the spirit portals at the North and South Poles to merge, while an aura of spirit energy envelops the Earth. During this event, Raava and Vaatu must engage in a battle that determines the fate of the world until the next Harmonic Convergence. However, after Raava merged with Wan, becoming the Avatar Spirit, it became the Avatar's duty to battle Vaatu in the Spirit World near the two portals that connect the two poles of the physical world.